There are many ways but the simplest is the following. What you do is create a collection within Classic that you sync with the cloud (control or right click the collection and select "sync with Lightroom"). Then from the submenu just below that (Lightroom links) select "Make collection public".
You can now copy the link from the same sub menu and send it to people. They can then follow that link and add comments or stars if they like the photo. Those comments will sync to Classic and you can find them in the comments section in the panel on the right.